Shape-selective catalysis is described, e.g., by N. Y. Chen, W. E. Garwood, and F. G. Dwyer, Shape Selective Catalysis in Industrial Applications, 36, Marcel Dekker, Inc. (1989). Within a pore of the molecular sieve, hydrocarbon conversion reactions such as isomerization, disproportionation, alkylation, and transalkylation of aromatics are governed by constraints imposed by the pore size. Reactant selectivity may occur when a fraction of the feedstock is too large to enter the molecular sieve pores to react, while product selectivity may occur when some of the products cannot leave the molecular sieve pores. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the molecular sieve pores or cages.
Another type of selectivity results from configurational constraints on diffusion where the dimensions of the molecule approach that of the molecular sieve pore system. A small change in the dimensions of the molecule or the molecular sieve pore can result in large diffusion changes leading to different product distributions. This type of shape-selective catalysis is demonstrated, for example, in selective alkyl-substituted benzene disproportionation to para-dialkyl-substituted benzene.
A representative para-dialkyl-substituted benzene is para-xylene. Typical methods for the production of para-xylene include the methylation of toluene and the disproportionation of toluene over a catalyst under conversion conditions. Such methods may result in the production of a mixture of the three xylene isomers, i.e., para-xylene, ortho-xylene, and meta-xylene. Depending upon the degree of selectivity of the catalyst for para-xylene (para-selectivity) and the reaction conditions, different percentages of para-xylene are obtained. Of the xylene isomers, i.e., ortho-, meta- and para-xylene, para-xylene is of particular value as a large volume chemical intermediate in a number of applications, such as the manufacture of terephthalic acid, which is an intermediate in the manufacturer of polyester.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts. One such method involves selectivating the catalyst, e.g., ZSM-5, with a selectivating agent. The term “selectivating agent” is used herein to indicate substances which will increase the shape-selectivity (e.g., para-selectivity) of the catalyst. For example, one technique, as disclosed in U.S. Pat. No. 5,243,117, involves treating the catalyst with a selectivating agent containing silicon. This technique usually requires several sequential silicone treatments that can substantially increase the cost of manufacturing the catalyst. Another technique, as disclosed in U.S. Pat. No. 4,097,543, involves the selective disproportionation of toluene in the presence of a catalyst comprising a molecular sieve, e.g., ZSM-5, that contains a controlled amount of carbon coke deposited on the catalyst. This technique requires on-stream selectivation of the catalyst and further selectivations after regeneration of the catalyst. Still another technique involves impregnating the catalyst with oxides that are difficult to reduce, such as those of magnesium, calcium, and/or phosphorus.